276 CHAPTER 11Clearly, life spans can evolve. It is possible that senescence does not occur in
some clonally propagated organisms that retain primordial stem cells, which can
differentiate into the organism’s diverse cell types [45]. But in most multicellular
species, potential life span is one of several components of an organism’s life his-
tory that are intimately related to fitness, and that have evolved by natural selec-
tion. Fecundity (number of offspring) likewise varies. Many bivalves and other
marine invertebrates release thousands or millions of tiny eggs in each spawn-
ing, but a blue whale (Balaenoptera musculus) gives birth to a single offspring that
weighs as much as an elephant, and a kiwi (Apteryx) lays a single egg that weighs
25 percent as much as its mother (FIGURE 11.2). Many species, such as humans,
reproduce repeatedly; others, such as century plants (Agave) and some species of
salmon (Oncorhynchus), reproduce only once and then die. Reproductive age may
be reached rapidly or slowly. A newly laid egg of Drosophila melanogaster may be a
reproducing adult 10 days later, and a parthenogenetic aphid may carry an embryo
even before she herself is born. In contrast, periodical cicadas feed underground for
13 or 17 years before they emerge, reproduce, and then die within a month.
What accounts for such extraordinary variation in species’ survival and reproduc-
tion—the very features we would expect to be most intimately related to their fitness?
In this chapter we seek to understand how natural selection has shaped life history
traits (those that affect rates of survival and reproduction at each age) and organisms’
ecological niches (the range of conditions they live in and resources they use). These
topics are themes in evolutionary ecology, the study of how evolution has shaped the
interactions between organisms and their environment.Life History Traits as Components of Fitness
A mutation that increases fecundity or survival will increase individual fitness,
as long as it has no other effects, and will therefore become fixed in the popula-
tion. At first surmise, then, we should expect any species to evolve ever-greater
fecundity and an ever-longer life span. The challenge, therefore, is to understandFIGURE 11.1 pecies vary greatly in life span. S
(A) Bristlecone pines (Pinus longaeva), surviv-
ing in the punishing environment of desert
mountaintops in California, are among the
oldest known individual organisms. (B) Draba
verna, a member of the mustard family, is an
annual plant that germinates in early spring,
sets seed within a few months, and dies.
(C) Asexually propagating corals may not age,
and persist for thousands of years. (D) Some
rotifers live for only a few weeks.Futuyma Kirkpatrick Evolution, 4e
Sinauer Associates
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